docs: def get_applicable_variables(self, ds):
          '''
          Returns a list of variable names that are applicable to ACDD Metadata
          Checks for variables. This includes geophysical and coordinate
          variables only.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          '''
          if self._applicable_variables is None:
              self.applicable_variables = cfutil.get_geophysical_variables(ds)
              varname = cfutil.get_time_variable(ds)
              # avoid duplicates by checking if already present
              if varname and (varname not in self.applicable_variables):
                  self.applicable_variables.append(varname)
              varname = cfutil.get_lon_variable(ds)
              if varname and (varname not in self.applicable_variables):
                  self.applicable_variables.append(varname)
              varname = cfutil.get_lat_variable(ds)
              if varname and (varname not in self.applicable_variables):
                  self.applicable_variables.append(varname)
              varname = cfutil.get_z_variable(ds)
              if varname and (varname not in self.applicable_variables):
                  self.applicable_variables.append(varname)
  
          return self.applicable_variables